Various processes are known for preparing tetrachloropyridine from pentachloropyridine. For example, U.S. Pat. No. 4,259,495 discloses a process whereby symmetrical tetrachloropyridine is obtained from pentachloropyridine via zinc reduction in a dialkylalkanephosphonate solvent in the presence of an ammonium salt of an inorganic acid. The process is also described in J. Hetero. Chem., 17, 493 (1980). In U.S. Pat. No. 3,993,654 is described a process for preparing tetrachloropyridine by heating together pentachloropyridine, a mineral or organic hydrogen donating acid and an oxidizable metal such as zinc which combines readily with reactive chlorine.
These known processes suffer the disadvantages in that they require the presence of flammable, toxic or otherwise corrosive substances, including hydrochloric acid, in order to effect the reaction. The use of such substances, in turn, effects a chemical reduction of zinc by the added mineral acid or base. Such chemical reductions can cause reductions in yield due to undesirable by-product formation, such as the dichloropyridines and the trichloropyridines.